Monsoon Season Sector: Embedded CB, Icing and a Forecast Deterioration
It is late July and you are dispatched on a domestic sector in the monsoon region. The route crosses a trough axis and the forecast shows a moist southwesterly flow with a low-level jet of 40 kt at 5,000 ft. The area forecast mentions widespread embedded CB with tops to FL400, moderate to severe icing in CB, and a SIGMET for severe turbulence below FL250. Your aircraft is a non-FBW turboprop with a service ceiling of FL250 and no radar. The destination has a TAF of TEMPO 2,000 m visibility with heavy rain and BKN 800 ft. The sector is 2.5 hours with no en-route alternate within 200 NM.
1. Assess the hazard from the low-level jet and the embedded CB, and decide whether the flight can be conducted as planned.
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The low-level jet of 40 kt at 5,000 ft in a moist southwesterly flow produces strong low-level wind shear and a favourable environment for organised convection. The embedded CB means the cells are hidden within layer cloud, so without radar you cannot see and avoid them visually. The combination of embedded CB, severe turbulence below FL250 and a service ceiling of FL250 means the entire usable altitude band is exposed to the hazard, and you cannot climb above it.
Decision: the flight as planned is not acceptable. A non-radar aircraft in embedded CB with no en-route alternate within 200 NM has no safe escape route if a cell develops on track. The correct decision is to delay, reroute along a path with radar coverage and a closer en-route alternate, or cancel. If the flight must proceed, it should be at an altitude and route chosen to remain clear of the trough axis and the jet, with a firm diversion plan, but the lack of an en-route alternate is the decisive factor.
2. What does the destination TEMPO of 2,000 m visibility with heavy rain and BKN 800 ft mean for your approach planning, and how would icing affect your options?
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A TEMPO of 2,000 m visibility with heavy rain and BKN 800 ft means temporary deteriorations to below a typical 5 km visibility and a cloud base at 800 ft. Heavy rain reduces visibility and can produce a wet runway and aquaplaning risk. The BKN 800 ft is above a typical 200 ft DA, but the heavy rain may reduce visibility below the approach minima temporarily, and the TEMPO conditions may coincide with your arrival.
Icing: moderate to severe icing in CB is forecast. In a non-FBW turboprop without radar, entering a CB at any level risks rapid airframe icing, which degrades performance and can be catastrophic if the aircraft is not adequately protected. The icing forecast reinforces the need to avoid the cells entirely rather than press through them. Plan to carry extra fuel for a diversion, brief the possibility of a go-around in heavy rain, and treat any encounter with icing as a reason to change level or route immediately.